The present invention relates to heat conducting laminates, and more particularly, to laminates having layers of metal and reinforced polymer matrix composite.
Heat sinks and other heat dissipating plates defined broadly herein as heat transfer devices, for electronic components and the like, are generally made from metals or metal alloys which have good thermal conductivity to permit good heat transfer from electronic components and devices, such as diodes and transistors. High temperatures resulting from the heavy current in diodes and heat generated in other electronic components require, in many cases, that heat sinks be used to draw the heat away from the electronic element or component. When heat sinks are not employed, various other devices have been designed to be attached directly to a heat dissipating plate or a chassis, which in turn will act as the heat transfer element.
In many of the foregoing cases, when the diodes and other electronic elements or components are attached to the heat sink or heat dissipating plate or other heat transfer device, problems arise because of the different coefficient of thermal expansion (CTE) of the various elements, for example, the difference in the coefficient of thermal expansion of the heat sink and the electrically insulating substrate to which the diode or transistor is attached, or the difference in coefficient of thermal expansion between the diode and the heat sink, and the like. When there is a significant difference in coefficient of thermal expansion between the components, temperature changes arising from soldering, and heat generated in the systems and from ambient conditions can cause large thermal stresses, resulting in failure of the components. Aluminum and copper are most frequently used as heat sinks and heat dissipating plates because they have good thermal conductivities, but the coefficient of thermal expansion of aluminum and copper are so high that heat or cold result in the separation of the heat transfer device from the next adjacent element which is usually made from a material having a lower coefficient of thermal expansion.
No single monolithic metal has a low coefficient of thermal expansion and a high thermal conductivity. The semiconductors and other critical elements and components used in electronic components, circuits or systems, such as silicon and gallium arsenide, are brittle, have low coefficients of thermal expansion and generate considerable waste heat in operation. Consequently, the minimum requirements for heat transfer devices for these components in electronic systems are low coefficient of thermal expansion and high thermal conductivity, and in most instances, such as in those cases where light weight is important such as space applications, low density is also a key consideration. Composite materials made from various substrates and organic polymer materials, such as laminates made of a paper substrate and a phenolic resin or a glass substrate and epoxy resin, as well as ceramic material such as alumina plates, have heretofore been used as substrates for printed wiring boards and heat sinks. However, the prior art suggests that these substrate materials are defective because of their low thermal conductivity and because of their inadequacy in transferring heat generated from such elements as integrated circuits, large-scale integrated circuits (LSI), power diodes and the like. Other laminates have also been developed in the form of laminates consisting of a metal base and an organic polymeric material or metal-ceramic composite plates, such as an electrical insulating alumilite film formed on an aluminum plate, however, these laminates are inadequate because of the thermal resistance due to the presence of the organic polymeric material or to the cracking of the alumilite film. Furthermore, in the foregoing materials, it is difficult to match or control the coefficient of thermal expansion of the plate.
In U.S. Pat. No. 4,307,147, there is described a thermally conductive and electrically insulating substrate which has a film composed of a dispersion of metal oxide particles with specified polyhedral shapes having specified shape factors in an adhesive organic polymer on a thermally conductive metal plate. In U.S. Pat. No. 4,307,147, the polyhedral-shaped metal oxide particles as well as the prior art irregularly-shaped particles, must be positioned on the metal plate in a face-to-face contact therewith. The thermally conductive and electrically insulating substrate of U.S. Pat. No. 4,307,147 used for the manufacture of wiring boards or heat discharge plates and the like is disadvantageous because it merely loosely positions metal oxide having flat surfaces or prior art metal oxides having irregularly-shaped surfaces against a metal plate and glues or fixes the metal oxide particles on the metal surface. Furthermore, the substrate formed in U.S. Pat. No. 4,307,147 is disadvantageous because it is difficult to match the coefficient of thermal expansion of the substrate to an adjacent component due to the positioning of the particles merely on the surface of the metal plate.
The matching of the coefficient of thermal expansion of the substrate to the next adjacent component in electronics devices is critical because it prevents structural and electrical failure during thermal cycling over the operational range of the components. Accordingly, it is desirable to match the coefficient of thermal expansion of the heat transfer device, such as a heat sink, to the coefficient of thermal expansion of the element or elements to which it is attached, and it is desirable to provide laminates which not only have excellent thermal conductivity but which also provide the capability of matching the coefficient of thermal expansion of adjacent elements
From the foregoing, it can be seen that there is a need for heat sinks, heat dissipating plates and other heat transfer devices which have a low coefficient of thermal expansion and/or a tailored coefficient of thermal expansion as well as a high thermal conductivity.